Nfs vs iSCSI

[Uzzi]

Hi
I've 3 separate Proxmox host (different release)
I've 1 NAS (I'm evaluating FreeNas and Openmediavault as OS) for VMs
I've 1 Qnap for Backup and Iso

Now I've set Nfs protocol for backup, I'm uncertain about NFS or iSCSI protocol for VMs

Can anyone recommend me best solution?

(My future project is to normalize differnts proxmox releases on differents host and clustering its)

Thansk